This application is a continuation-in-part of patent 4,853,595, filed on Aug. 31, 1987 in the names of Robert R. Alfano and Ardie D. Walser.
The present invention relates generally to an ultrafast sampling oscilloscope and more particularly to an oscilloscope which includes a photomultiplier tube which is constructed and which possesses the sensitivity to time resolve a picosecond voltage pulse with femtosecond resolution.
An oscilloscope is a well known type of cathode ray tube device which produces a visible pattern which is the representation of an electrical input signal.
A photomultiplier tube is a well known type of photosensitive device that is commonly used in electro-optical systems to time resolve optical signals.
Basically, a photomultiplier tube comprises a photocathode, an electron multiplier and an anode, all disposed in an evacuated glass housing, with potential differences set up between the electrodes and the electron multiplier to cause photoelectrons emitted by the photocathode to pass through the electron multiplier and on to the anode.
When light strikes the photocathode, photoelectrons are emitted into the vacuum in proportion to the intensity of the light. These photoelectrons are multiplied by the electron multiplier and then collected by the anode as an output signal.
Because of the electron multiplication, photomultiplier tubes are uniquely sensitive among photosensitive devices currently used to detect radiant energy in the ultraviolet, visible, and near infrared regions. Photomultiplier tubes also feature relatively fast time response and low noise.
The photocathode in a photomultiplier tube is generally arranged in either a side-on or a head-on configuration. In the side-on type configuration a generally circularly shaped photocathode receives incident light through the side of the glass housing while, in the head-on type, a generally cylindrically shaped photocathode receives light through the end of the glass housing. In general, the side-on type photomultiplier tube is widely used for spectrophotometers and general photometric systems. Most of the side-on types employ an opaque photocathode (reflection-mode photocathode) and a circular-cage structure electron multiplier which has good sensitivity and high amplification at relatively low supply voltage.
The head-on type photomultiplier tube has a semitransparent photocathode (transmission-mode photocathode) deposited upon the inner surface of the entrance window while in the side-on type, the photocathode is a separate structure. Because the head-on type provides better uniformity and lower noise, it is frequently used in a scintillation detection and photon counting applications.
The electron multiplier in a photomultiplier tube is usually either a series of electrodes, called dynodes, or a microchannel plate. As is known, a microchannel plate (MCP) is a form of secondary electron multiplier consisting of an array of millions of glass capillaries (channels) having an internal diameter ranging from 10 um to 20 um fused into the form of a thin disk less than 1 mm thick. The inside wall of each channel is coated with a secondary electron emissive material having a proper resistance and both ends of the channel are covered with a metal thin film which act as electrodes. Thus, each channel becomes an independent secondary electron multiplier.
When a voltage is applied between both sides of an MCP, an electric field is generated in the direction of the channel axis. When an electron hits the entrance wall of the channel, secondary electrons are produced. These secondary electrons are accelerated by the electric field and travel along parabolic trajectories determined by their initial velocity. Then they strike the opposite wall and produce other secondary electrons. This process is repeated many times along the channel and, as a result, the electron current increases exponentially towards the output end of the channel.
In an article entitled High Speed Electrical Sampling by Photomission in Appl. Phys. Lett. 49(6) 11 Aug. 1986 by R. B. Marcus et al pages 357-359 there is disclosed a method for contactless temporal sampling of high speed electrical signals using spectral analysis of photoelectrons emitted when a signal-carrying conductor is illuminated by ultrashort light pulses.
In an article entitled High Speed Circuit Measurements Using Photoemission Sampling in Appl. Phys. Lett. 49, (4) 28 July 1986, pages 226-228 by J. Bokor et al there is disclosed a method for measuring voltage waveforms on metallization lines of an integrated circuit or an electronic device which is capable of picosecond resolution.
In U.S. Pat. No. 3,885,178 to Goehner there is disclosed a photomultiplier tube (PMT) which converts a received light signal to an output electrical signal of substantially greater intensity by employing a photocathode to convert incident light to free electrons, a plural dynode accelerating structure for effectively multiplying the free electrons, and an impact ionization diode (IID) for further multiplying and collecting the free electrons to provide a corresponding electrical output signal. The PMT can be an electrostatic device, in which the photocathode and the dynodes are mounted in opposed staggered positions, or a static crossed field device, in which the photocathode and the dynodes all are mounted opposite an accelerating rail and a magnetic field is provided to urge the electrons laterally along the tube. The IID's junction is reversed biased and the entire diode is maintained at a substantially higher potential than the last dynode. The PMT can be gain controlled or turned off without affecting dynode potentials by controlling the IID's potential. Due to the gain provided by the IID, dynode current can be greatly reduced, thereby increasing substantially the life of the tube without affecting its overall gain.
Known patents of interest include U.S. Pat. No. 3,867,662 to Endriz; U.S. Pat. No. 3,914,136 to Kressel; U.S. Pat. No. 4,467,189 to Tsuchiya and U.S. Pat. No. 4,659,921 to Alfano.
One of the limitations of most prior art photomultiplier tubes is that although they have a relatively fast time response, they are not capable of time resolving events in the picosecond or femtosecond range.
It is an object of the present invention to provide an ultrafast sampling oscilloscope.
It is an object of this invention to provide an oscilloscope which is capable of time resolving picosecond test voltage pulses with femtosecond resolution.
It is still another object of the present invention to provide an oscilloscope which employs a photomultiplier tube which is constructed and which possesses the sensitivity to time resolve a picosecond voltage pulse with femtosecond resolution.